1. Field of the Invention
This invention relates generally to a diesel fuel composition and more particularly concerns a diesel fuel composition comprising dimethyl ether, methanol and water.
2. Description of the Prior Art
As alternatives to conventional hydrocarbon diesel fuel produced by refining petroleum, other liquid fuels obtained by the conversion of methane or coal have been under consideration since the 1920's. Methanol has been proposed as one such alternative fuel for internal combustion engines. Methanol is usually manufactured from carbon monoxide and hydrogen, which have historically been obtained in large volume from either natural gas or coal. Carbon monoxide can also be obtained from almost any carbon-containing substance, including agricultural and forest products and many waste materials. The large supply and wide distribution of raw materials for manufacturing methanol is responsible to a large degree for its growing use as a fuel for internal combustion engines. However, methanol has a very low heating or BTU value. Thus, the performance of an internal combustion engine declines considerably when methanol is employed as the fuel.
By contrast, relative to methanol, dimethyl either has a higher BTU value and is nontoxic. In addition, dimethyl ether is a clean-burning fuel whose combustion gases are essentially free of solid particles. Numerous methods have been disclosed for the production of dimethyl ether in combination with methanol and water from synthesis gas obtained from various sources, such as natural gas, coal or essentially any carbon-containing substance. Bell et al., U.S. Pat. No. 4,341,069; Van Dijk et al., U.S. Pat. No. 5,177,114; and published European Patent Applications Nos. 0324475 and 0409086 A1 are examples of such disclosures. In particular, European Patent Applications Nos. 0324475 and 0409086 A1 disclose how process conditions can be controlled in one such method in order to produce mixtures of dimethyl ether and methanol having a wide range of mole ratios of dimethyl ether to methanol.
In numerous methods for the manufacture of dimethyl ether, dimethyl ether is produced in a product mixture that also contains methanol and/or water. Furthermore, removal of methanol and water from dimethyl ether in such a product mixture would require additional processing steps. Thus, it would be highly desirable to be able to employ mixtures of dimethyl ether, methanol and water--or, in other words, crude or unpurified dimethyl ether--directly as diesel fuels in order to avoid the aforesaid additional processing steps associated with purifying crude dimethyl ether and ideally so that process conditions could be employed in order to produce such mixtures directly from synthesis gas. In that way, it would be possible to avoid or at least minimize the need for additional processing steps, such as purification steps, and still produce a highly effective and economical alternative diesel fuel.
There have also been a number of disclosures of diesel fuels that comprise either a mixture of dimethyl ether and methanol, a mixture of dimethyl ether, methanol and water, or a mixture of dimethyl ether and a cetane number-improving additive. For example, German Patent No. 654,470 (1937) discloses mixtures of dimethylether and methanol containing from 5 percent to 45 percent of methanol (and hence from 55 percent to 95 percent of dimethyl ether) for use as fuels for an internal combustion engine, which permits the relatively high heating value of dimethyl ether to be utilized while avoiding an excessive penalty due to the tendency of dimethyl ether to knock in a spark ignition engine.
Furthermore, Norton, U.S. Pat. No. 4,422,412, discloses a diesel fuel composition that contains a mixture of dimethyl ether, methanol and water containing "up to about 50%, e.g., about 5 to 30%" of dimethyl ether. This mixture is produced by the catalytic conversion of methanol to dimethyl ether and water in a reactor whose outlet is in communication with a cylinder of an internal combustion engine.
In addition, Norton et al., U.S. Pat. No. 4,603,662, discloses a diesel fuel composition that contains a mixture of at least one ether and at least one alcohol, and optionally additional constituents such as water or cetane improvers, which contain "from 5 to 80%, more usually from 5 to 20% by volume of ethers in the fuel." The patent discloses that a fuel containing a combination of dimethyl ether and methanol is a particularly convenient fuel and specifically illustrates this combination with blends containing: (a) 95 percent of methanol and 5 percent of dimethyl ether by volume in Examples 1 and 3; and (b) 78 percent of methanol and 20 percent of dimethyl ether in Example 5.1. The patent also contains the following specific illustrations of blends of alcohols and ethers that contain at least 50 percent of the ethers: (a) 80 percent of isoamyl ether and 20 percent of methanol in Example 5.9; (b) 60 percent of di-n-propyl ether, and 40 percent of methanol in Example 6.2; and (c) 60 percent of di-n-butyl ether and 40 percent of methanol in Example 6.5.
Levine, U.S. Pat. No. 4,892,561, discloses a first diesel fuel composition that contains 95-99.9 percent by weight of dimethyl ether and 0.1-5 percent by weight of a cetane number-improving additive. This patent also discloses a second diesel fuel composition that contains at least 50 percent by weight of the aforesaid first diesel fuel and the remainder conventional hydrocarbon diesel fuel.
However, thus far there has not been a disclosure of the compositions of mixtures of dimethyl ether, methanol and water that contain the balance of concentration levels of dimethyl ether, methanol and water that is necessary for the resulting diesel fuel to afford both environmental benefits and good ignition characteristics, that can be produced economically without the need for costly purification steps and that can be maintained as a stable single liquid phase both in use and during storage.